Here's the hard question: if you agreed with the anti-abortionists' claim that abortion is murder, would you think the ad was effective?

Climate change is also a complex issue, but I can see liking a billboard that raises awareness of the dangers using a basic concept with strong emotional impact. After all, advertising is all about reducing complex ideas to simple messages.

Good point, but bad example ( for me). I believe that climate change needs less "you piece of shiit" emotion and more practical and personal motivation and discussion. The emotion is preaching to the choir. We need converts.

I know you wouldn't do anything this tacky, but what about using (say) cute baby polar bears, with some kind of doom-ish fate looming over them? That's roughly the same approach.

It wouldn't work on me, because I know that those baby polar bears will grow into ruthless killing machines who would stalk me for days, torture me to death, and then eat me if my plane ever went down in the north, but then, I'm probably not the target audience for cute baby animal posters anyway.